It blows fuel into the airbox because the intake is the only place for the compression to escape if you take out the plug you have created anothe escape route, therefore it doesn't exhaust fuel into the airbox. I'm going to say you do have a timing issue even though the dealer did the valve adjustment, nobody is perfect even the dealer can make a mistake.
I think I would take off the valve covers and check for myself the valve adjustment. get it to TDC and loosen the valves up and readjust them to spec and see what you got then. If that doesn't work you may want to look at the carb for issues. Like Saber asked how long has it sat and did it just start this issue, and does it fire the plug?